Steel pipes made of carbon steel and low-alloy steel are widely used for steel pipes used for transportation of petroleum and natural gas. In recent years, there is an increasing tendency that extracted petroleum or natural gas contains wet carbon dioxide gas and hydrogen sulfide, and a problem arises in that the carbon dioxide gas and the hydrogen sulfide cause severe corrosion of the steel pipes.
In one technique developed to address the above problem, a corrosion inhibitor is added in the course of production of a steel pipe to prevent corrosion. However, the addition of the corrosion inhibitor not only results in an increase in the production cost of the steel pipe but also causes a new problem in that environmental pollution occurs due to the corrosion inhibitor. Accordingly, there is a need for a technique for improving the corrosion resistance of steel pipes without addition of the corrosion inhibitor.
A technique for producing a steel pipe made of duplex stainless steel has been investigated. The duplex stainless steel is a well-known material excellent in corrosion resistance, and the technique for producing a steel pipe made of the duplex stainless steel is a technique for improving the corrosion resistance of the steel pipe by utilizing the properties of the duplex stainless steel. However, since the duplex stainless steel is expensive, there is still a problem in that the production cost of the steel pipe increases.
In one steel pipe production technique investigated to address the above problem, low-carbon martensitic stainless steel that is lower in cost than the duplex stainless steel and has moderate corrosion resistance is used as the raw material of the steel pipe.
For example, Patent Literatures 1 and 2 disclose low-carbon martensitic stainless steel pipes which contain reduced amounts of C and N and about 11 to 14% Cr by mass and to which an austenite-stabilizing element is added. Since these steel pipes contain reduced amounts of C and N, they have an advantage in that their weldability is improved.
Generally, when a pipeline is constructed, ends of steel pipes are butted against one another and subjected to circumferential welding to thereby connect a large number of steel pipes. With the technique described in Patent Literatures 1 and 2, weldability is improved, so that preheating before the circumferential welding and heat treatment after the circumferential welding (hereinafter referred to as post-welding heat treatment) can be omitted. This can improve the working efficiency of the circumferential welding.
As for the working efficiency of the circumferential welding, it is necessary to improve the working efficiency of the circumferential welding in pipeline laying work to complete the work in a short time. In particular, when a submarine pipeline is laid, circumferential welding is performed on a pipe-laying vessel. Since the cost of the pipe-laying vessel is high, it is very important to complete the circumferential welding in a short time. There is therefore a need for a welding material that requires no preheating and no post-welding heat treatment.
When line pipes are laid in a cold region, it is necessary to use a welding material for obtaining a weld metal that exhibits excellent toughness at low temperature (e.g., −40° C.).
In this regard, welding materials (so-called welding wires) including various components have been practically used for circumferential welding of steel pipes.
When a martensitic stainless steel-made welding material is used for circumferential welding of the low-carbon martensitic stainless steel pipes disclosed in Patent Literatures 1 and 2, the weld metal is very hard, and this causes deterioration in toughness. Therefore, preheating and post-welding heat treatment must be performed, and this causes a problem in that the working efficiency of the circumferential welding is reduced.
When an austenitic stainless steel-made or Ni-based super alloy-made welding material is used for circumferential welding of the low-carbon martensitic stainless steel pipes disclosed in Patent Literatures 1 and 2, a problem arises in that the strength of the weld metal is likely to be lower than the strength of the base metal (so-called undermatching occurs). When a welding material made of 22Cr duplex stainless steel (22 mass % Cr-6 mass % Ni-3 mass % Mo) is used, it is difficult for the welding metal to ensure general X80-grade strength required of the low-carbon martensitic stainless steel pipes. When a welding material made of 25Cr duplex stainless steel (25 mass % Cr-7 mass % Ni-4 mass % Mo) is used, the X80-grade strength can be obtained because the strength of the weld metal is more increased than the strength of the base metal (so-called overmatching occurs). However, the cost of the circumferential welding work increases.
Generally, a reduction in strength of duplex stainless steel due to an increase in temperature is larger than that of martensitic stainless steel. Therefore, when the above-described 25Cr duplex stainless steel-made welding material is used, although the weld metal is overmatched at room temperature, undermatching is likely to occur at 100 to 200° C. When a duplex stainless steel-made welding material is used, selective corrosion may occur because of the difference in components between the weld metal and the base metal.
Under the above circumstances, a technique has been investigated, which allows circumferential welding of low-carbon martensitic stainless steel pipes to be performed efficiently (i.e., allows preheating and post-welding heat treatment to be omitted) and can be used to obtain a weld metal having high strength, high toughness, and high corrosion resistance.
For example, Patent Literatures 3 and 4 disclose welding materials suitable for circumferential welding of low-carbon martensitic stainless steel pipes.
When the welding material disclosed in Patent Literature 3 is used to perform circumferential welding, the Charpy absorbed energy of the weld metal at 0° C. is about 100 J, and therefore the toughness of the weld metal is insufficient for use in pipelines laid in a cold region. Moreover, post-welding heat treatment for 1 hour or longer is necessary, and a reduction in working efficiency is unavoidable.
When the welding material disclosed in Patent Literature 4 is used to perform circumferential welding, a fracture transition temperature of −40° C. or lower can be obtained without preheating and post-welding heat treatment, and the effect of improving low-temperature toughness is recognized. However, the maximum absorbed energy (so-called upper shelf energy) of the weld metal is significantly lower than that of the low-carbon martensitic stainless steel pipes. Therefore, to use this welding material for pipelines in a cold region, there is a need for further improvement in toughness.